Microfluidic chip with mixed porosities for reservoir modeling

ABSTRACT

Spherical grains and sacrificial particles are mixed in a suspension. The sacrificial particles are larger than the spherical grains. The suspension is injected into a channel in a microfluidic chip, and the spherical grains form microporous structures in the channel. The microporous structures are sintered in the channel. A solvent is injected into the channel, and the solvent dissolves the sacrificial particles and forms macropores between at least some of the microporous structures, thereby forming a mixed-porosity microfluidic chip.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional of and claims the benefit of priorityto U.S. patent application Ser. No. 17/155,619, filed Jan. 22, 2021, thecontents of which are incorporated by reference herein.

BACKGROUND

Hydrocarbons (for example, oil, natural gas, or combinations of them)entrapped in formations can be raised to the surface, that is, produced,using wells formed through the formations. Usually, the hydrocarbons areentrapped in the formations under pressure sufficient to flow thehydrocarbons through pores of the formations into the wells. Formationscan be of different types, for example, carbonate or sandstone, and canhave different porosities that affect the flow of the hydrocarbonsthrough the formations. Modeling fluid flow through reservoirs allowsfor improving efficiency in extracting hydrocarbons from reservoirs.

SUMMARY

An embodiment disclosed herein provides a method for modeling areservoir with a microfluidic chip having mixed porosities.

Certain aspects of the subject matter described herein can beimplemented as a method including mixing spherical grains andsacrificial particles in a suspension. The sacrificial particles arelarger than the spherical grains. The suspension is injected into achannel in a microfluidic chip, and the spherical grains formmicroporous structures in the channel. The microporous structures aresintered in the channel. A solvent is injected into the channel, and thesolvent dissolves the sacrificial particles and forms macropores betweenat least some of the microporous structures, thereby forming amixed-porosity microfluidic chip.

An aspect combinable with any of the other aspects can include thefollowing features. The mixed-porosity microfluidic chip is used tomodel a subsurface reservoir.

An aspect combinable with any of the other aspects can include thefollowing features. The reservoir is a carbonate reservoir characterizedby bi-modal porosity.

An aspect combinable with any of the other aspects can include thefollowing features. Modeling the subsurface reservoir includes studyingrock-fluid interactions.

An aspect combinable with any of the other aspects can include thefollowing features. Modeling the reservoir includes spectroscopicstudies of interactions between fluids and surfaces.

An aspect combinable with any of the other aspects can include thefollowing features. Modeling the subsurface reservoir includes studyingoil-water phase behavior in the pores of the mixed-porosity microfluidicchip.

An aspect combinable with any of the other aspects can include thefollowing features. The microfluidic chip is an optically transparent ortranslucent chip.

An aspect combinable with any of the other aspects can include thefollowing features. The spherical grains are calcium carbonate spheres.

An aspect combinable with any of the other aspects can include thefollowing features. The method further includes synthesizing the calciumcarbonate spheres.

An aspect combinable with any of the other aspects can include thefollowing features. The sacrificial particles are sodium chloridecrystals and the solvent is water.

An aspect combinable with any of the other aspects can include thefollowing features. The microporous structures comprise microporesbetween spherical grains, and the average width of the macropores is atleast about ten times larger than the average width of the micropores.

An aspect combinable with any of the other aspects can include thefollowing features. The calcium carbonate spheres are from about 25nanometers to about 25 microns in diameter.

An aspect combinable with any of the other aspects can include thefollowing features. The sodium chloride crystals have a width of about10 microns to about 250 microns.

Certain aspects of the subject matter described here can be implementedas a mixed-porosity microfluidic chip that includes a microchanneletched in a substrate. The microporous structures comprising calciumcarbonate spheres sintered in the microchannel. The chip furtherincludes macropores between at least some of the microporous structures.The average width of the macropores is at least about ten times largerthan the average diameter of the calcium carbonate spheres.

An aspect combinable with any of the other aspects can include thefollowing features. The microporous structures comprise microporesbetween the calcium carbonate spheres, and the average width of themacropores is at least about ten times larger than the average width ofthe micropores.

An aspect combinable with any of the other aspects can include thefollowing features. The calcium carbonate spheres are from about 25nanometers to about 25 microns in diameter.

An aspect combinable with any of the other aspects can include thefollowing features. The macropores have a width of about 10 microns toabout 250 microns.

An aspect combinable with any of the other aspects can include thefollowing features. The mixed-porosity microfluidic chip is opticallytransparent or translucent.

An aspect combinable with any of the other aspects can include thefollowing features. The microchannel has a width of from about 500microns to about 1500 microns and a height of from about 50 microns toabout 500 microns.

An aspect combinable with any of the other aspects can include thefollowing features. The macropores are substantially cubic in shape.

The details of one or more implementations of the subject matter of thisdisclosure are set forth in the accompanying drawings and thedescription. Other features, aspects, and advantages of the subjectmatter will become apparent from the description, the drawings, and theclaims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing of an exemplary microfluidic chip having straightchannels suitable as a component in an embodiment of the presentdisclosure.

FIG. 2A is a process flow diagram of a method for fabricating amicrofluidic chip having mixed porosities in accordance with anembodiment of the present disclosure.

FIG. 2B is a schematic diagram of a method for fabricating amicrofluidic chip having mixed porosities in accordance with anembodiment of the present disclosure.

FIG. 3A-3C are scanning electron micrographs of synthesized calciumcarbonate spheres in accordance with an embodiment of the presentdisclosure.

FIG. 3D is a Raman spectrum of synthesized calcium carbonate sphere inaccordance with an embodiment of the present disclosure.

FIG. 4A-4C are scanning electron micrographs of synthesized sodiumchloride crystals in accordance with an embodiment of the presentdisclosure.

DETAILED DESCRIPTION

To increase oil recovery efficiency, it is important to betterunderstand multiphase fluid behaviors and interactions amongoil-water-rock phases in underground oil reservoirs.

Carbonate reservoirs hold a significant proportion of the world's oilreserves. In a carbonate reservoir, large quantities of crude oil may bestored in microscale or nanoscale pores, and may be difficult to recoverwith conventional methods. Furthermore, the porosity of some carbonatereservoirs may be complex.

Some carbonate reservoirs exhibit both microporosity (defined herein aspore diameters less than about 10 microns) and also macroporosity(defined herein as a pore diameter greater than about 10 microns). Insome reservoirs exhibiting both microporosity and macroporosity, theporosity can be distinctly bimodal, with microporosity pores about tensup to one thousand times tighter and smaller than the macroporositypores. For example, in some bimodal carbonates of the Arabian Peninsula,a majority of the porosity can be attributed to two distinct fractions:a first fraction attributable to pores of between about 100 and about1000 nm in diameter, and a second fraction attributable to pores of tensto hundreds of micrometers in diameter.

In the field of research about oil reservoirs and improved oil recovery(IOR) and enhanced oil recovery (EOR), it is desirable to have amicromodel that resembles the complicated porosities of naturalcarbonate reservoirs. Reservoir micromodels—sometimes referred to as“reservoir-on-a-chip”—have been used to mimic the undergroundoil-reservoir environment for multi-phase flow studies, enhanced oilrecovery, and reservoir network mapping. However, existing micromodelsmay be limited in their usefulness in modeling reservoirs that may havemultiple porosities in the same rock. Furthermore, typical micromodelsmade of glass or polymer materials may not be representative of thegeochemical surface of carbonate reservoir rocks.

Generally, in accordance with the embodiments described in the presentdisclosure, a microfluidic chip with mixed porosities can be fabricatedand utilized for such modeling purposes. The method of fabrication caninclude synthesizing nanoscale-sized calcium carbonate (CaCO₃) spheresand also micron-scale sacrificial particles, such as sodium chloride(NaCl) crystals. The calcium carbonate spheres and sacrificial particlesare mixed together in a suspension. The suspension is then injected intoa channel in a microfluidic chip, such that the calcium carbonatespheres form microporous structures in the channel, surrounding thesacrificial particles. After sintering, a solvent is injected into thechannel to dissolve the sacrificial particles, thus forming macroporesbetween the microporous structures.

The method allows for tuning of particle size(s) and resultingporosities, such that the resulting chip can more closely correspond tothe characteristics of natural carbonate reservoirs. The resulting chipcan be used to study oil-water phase behavior and rock-fluidsinteractions of matrices featuring both microporosity and macroporosity,with small volume of samples at low cost. The surface of the resultingchip is optically transparent or translucent and allows to directlyvisualize fluid behaviors near the surface by advanced spectroscopic andimaging techniques, providing useful information for enhanced oilrecovery.

FIG. 1 shows straight-channel microfluidic chip 150 of a type availablefrom Micronit Company of the Netherlands and provides a suitablecomponent for an embodiment of the present disclosure. Chip 150 has alength of about 45 mm, a width of about 15 mm, and a thickness of about1245 μm. Chip 150 is comprised of borosilicate glass and has threechannels 152, 154, and 156 having of width of 500 μm, 1500 μm, and 1000μm, respectively, with each channel having a height of 50 μm. In someembodiments, the height of the channels can vary from about 50 μm toabout 500 μm. Chip 150 is optically transparent and fits into a housing158 that can, in turn, fit into a chip holder (not shown). Particlescorresponding to natural sedimentary rock grains and other substancescan be injected into one or more of channels 152, 154, and 156 viainjection holes 160, 162, and 164, respectively, to form granular orother structures which resemble natural reservoirs in geometry,composition, or other characteristics. In accordance with an embodimentof the present disclosure, chip 150 can include mixed-porositystructures 269 comprising macropores between microporous structuressintered in one or more of the channels (for example, channel 154), asdescribed in more detail in FIGS. 2A and 2B.

Other embodiments of the present disclosure can utilize chips of othersuitable sizes, channel geometries, porosities, and othercharacteristics, available from various commercial suppliers.

FIG. 2A is a process flow diagram of a method 200 for fabricating amicrofluidic chip having mixed porosities in accordance with anembodiment of the present disclosure. FIG. 2B is a schematic diagramillustrating some of the steps of the method described in reference toFIG. 2A. The method of FIG. 2A will be described with reference to FIG.2B. FIG. 2B is not drawn to scale.

The method begins at block 202 with the synthesis of calcium carbonate(CaCO₃) spheres of a desired size. The calcium carbonate spheres can besynthesized in accordance with one or more of the methods described inthe Examples section below, or via another suitable method. In oneembodiment, the calcium carbonate spheres have a substantially uniformsize and a diameter of about 25 nm to about 25 μm.

The calcium carbonate spheres synthesized in accordance with block 202of FIG. 2A correspond to grains 260 at step 240 of FIG. 2B. Synthesizedcalcium carbonate grains can serve as a proxy for natural calciumcarbonate grains and thus are suitable for creating microfluidic chipsfor modeling of carbonate reservoirs. In other embodiments, for examplefor modeling of other reservoir types, particles or grains of anothercomposition (for example, dolomite (calcium magnesium carbonate,CaMg(CO₃)₂) or sandstone (silicon dioxide, SiO₂) particles) of aspherical or other shape (oblong, cubic, needle-shaped, or othersuitable shape) can be utilized for grains 260 instead of calciumcarbonate spheres. In some embodiments, grains 260 can comprise amixture of particle compositions and/or shapes. In some embodiments,particles of a suitable composition, shape, or size may be purchasedcommercially instead of synthesized.

At block 204 of FIG. 2A, sodium chloride crystals of a desired size aresynthesized. The sodium chloride (NaCl) crystals can be synthesized inaccordance with the methods described in the Examples section below, orvia another suitable method. In one embodiment, the sodium chloridecrystals with width of about 10 microns to about 50 microns.

The sodium chloride crystals synthesized in accordance with block 204 ofFIG. 2A correspond to sacrificial particles 262 at step 240 of FIG. 2B.Sodium chloride crystals are typically cubic in shape. In otherembodiments, sacrificial particles 262 can comprise particles of adifferent composition, readily dissolvable with a suitable solvent. Forexample, alternative sacrificial particles can include MX₂ (M=K, Na, Cs;X═Cl, Br, NO₃) or MX₂ (M=Zn, Cu, Co, Ni, Fe, Mn, Cd; X═Cl, NO₃), whichare water soluble. In some embodiments, sacrificial particles 262 cancomprise a mixture of particle compositions and/or shapes. In someembodiments, sacrificial particles of a suitable composition, shape, orsize may be purchased commercially instead of synthesized.

At block 206 of FIG. 2A, and as illustrated at step 242 of FIG. 2B, thegrains 260 are mixed with sacrificial particles 262 in the desired ratiointo a suspension 264. Suspension 264 comprises a suitable fluid phaseto carry grains 260 and sacrificial particles 262. The fluid phase cancomprise ethanol, acetone, hexane, chloroform, or another suitable fluidphase.

In one embodiment, the desired ratio of grains 260 to sacrificialparticles 262 depends on the amount of microporosity of the carbonatereservoir which the chip is intended to model. For example, in MiddleEast carbonate reservoirs it is common to have 20% microporosity, withvalues of microporosity up to 50% having been observed. Furthermore, asthe ratio of grains 260 to sacrificial particles 262 increases, themechanical stability of the resulting 3D microstructures (see below)tends to increase. In one embodiment of the present disclosure, to modela carbonate reservoir having about 25-30% microporosity would correspondto a volume ratio of about 2-to-1 of grains 260 to sacrificial particles262.

At block 208 of FIG. 2A, and as illustrated at step 244 of FIG. 2B, thesuspension 264 is injected into a channel 272 of a microfluidic chip270. Microfluidic chip 270 may correspond to chip 150 of FIG. 1 or maybe another suitable microfluidic chip. A paper filter can be placed atthe end of the channel to block the particles from flowing out. In oneembodiment, the suspension is injected continuously until the channel isfully packed with solid particles.

Settling within channel 272, grains 260 form granular structures 266having micropores 267 comprising the voids between the spheres andhaving a porosity dependent on grain shape and size. For example,calcium carbonate spheres can form granular structures comprising 3Dmicrostructures with random close packing (RCP). For spheres withsubstantially uniform sizes in a 3D RCP structures, the micropores 267are substantially tetrahedral or octahedral in shape and have a width ofabout 22.5% to 41.4% of the diameter of the spheres. For example, in oneembodiment, spheres with a diameter of about 25 nm to about 25 μm canform a granular structure 266 with micropores 267 with a width of about10 nm to about 10 Sacrificial particles 262 are distributed among andbetween granular structures 266.

At block 210 of FIG. 2A, chip 270 is placed in an oven and sintered. Inone embodiment wherein the grains 260 comprise calcium carbonatespheres, chip 270 is sintered at 250° C. for about two hours. At thistemperature, calcium carbonate spheres are immobilized to form a robustporous structure without substantially changing the size or morphologyof the spheres. Nitrogen gas is then flowed through channel 272 to drythe structures 266. The sintering process substantially immobilizes thespheres and stabilizes the structure so that it remains intact withoutcollapsing. The sintering process may also cause some cracks among theassembled calcium carbonate sphere structures due to slight shrinking ofthe spheres. In one embodiment, cracks induced by thermal shrinking ofcalcium carbonate spheres are of a similar or smaller than the size ofthe spheres. The chip is removed from the oven and allowed to cool toroom temperature.

At block 212 of FIG. 2A, deionized water or another suitable solvent isinjected into the channel. The deionized water dissolves the sodiumchloride crystals and flushes out the dissolved Na⁺ and Cl⁻ ions as wellother ionic impurities. At block 214, nitrogen (N₂) gas or anothersuitable drying medium is flowed through the channel to dry the particlestructures which remain in the channel.

Step 246 of FIG. 2B illustrates chip 270 after the solvent has beeninjected into chip 270 and the chip dried and made ready for use as areservoir micromodel. Grains 260 remain in granular structures 266. Asthe sacrificial particles 262 dissolve, macropores 268 are left in theirplace between and among granular structures 266. For example, in oneembodiment, sacrificial particles 262 having a width of about 10 μm toabout 250 μm can result in macropores 268 having a width of about 10 μmto about 250 μm. In an embodiment wherein grains 260 are nanoscale ormicroscale particles and the sacrificial particles are micron-scale, themacropores 268 between the granular structures 266 result in mixedporosity structures 269 within channel 272, with voids of macroporosity(from macropores 268 resulting from dissolution of sacrificial particles262) among and between the regions of microporosity (from micropores267) formed by the granular structures 266.

The resulting mixed-porosity microfluidic chip remains opticallytransparent or translucent, allowing interactions between fluids and thesurfaces to be directly visualized by multiple characterization tools,such as advanced spectroscopic and/or microscopic techniques, providinguseful information for enhanced oil recovery. By injecting oil, water,and other fluids into the chip, oil-water phase behavior and theinteractions between fluids and surfaces, such as rock-fluidinteractions, can be observed and studied.

The ratios, composition, sizes, and shapes of grains 260 and sacrificialparticles 262 can be tuned so as to result in a micromodel thatcorresponds to specific natural reservoirs. For example, in oneembodiment, grains 260 comprise calcium carbonate spheres with adiameter of about 500 nm to about 2500 nm and sacrificial particles 262comprise sodium chloride crystals with width of about 10 microns toabout 50 microns, with the suspension mixture comprising about 50%sodium chloride crystals (with the remainder of the suspension mixturecomprising the calcium carbonate spheres) and with resulting randomclose packing of granular structures 266. In such an embodiment, theresulting chip is characterized by a bimodal porosity similar to thatfound in, for example, the Arab-D formation of the Arabian Peninsula,with pore diameters in the granular structures 266 of between about 100and to about 1000 nanometers and macroporosity resulting from macropores268 (with widths corresponding to the width of the sacrificial particles262).

EXAMPLES Synthesis of Calcium Carbonate Particles and Calcium MagnesiumCarbonate Particles

Calcium carbonate crystals or particles can be chemically synthesizedthrough the following reaction:

CaCl₂+Na₂CO₃→CaCO₃(s)+2NaCl

Calcium magnesium carbonate crystals or particles can be chemicallysynthesized through the following reaction:

CaCl₂+MgCl₂+2Na₂CO₃→CaMg(CO₃)₂(s)+4NaCl

In an embodiment of the present disclosure, a solution precipitationmethod can be used to synthesize calcium carbonate or calcium magnesiumcarbonate particles with sizes varying from 20 nm-20 μm with a narrowsize distribution.

In a synthesis process in accordance with this embodiment, 20.3 gcalcium chloride (CaCl₂.6H₂O) and 14.7 g magnesium chloride (MgCl₂.2H₂O)was dissolved in 100 mL deionized (D.I.) water, one part of the solutionand 21.2 g sodium carbonate (Na₂CO₃) was dissolved in another part ofthe solution. The two parts of the solutions were rapidly mixed withvigorous stirring with a 1:1:2 molar ratio of Ca²⁺, Mg²⁺ to CO₃ ²⁻. Thereaction mixture was then transferred into an autoclave and heated at180° C. for 12 hours. After cooling down to room temperature, the formedparticles can be separated from the suspension by centrifuge and washedby water and ethanol in turn to remove impurities. Finally, the purifiedcalcium magnesium carbonate particles are redispersed in absoluteethanol. The shape of resulting calcium carbonate particles is sphericalwith a narrow size distribution. By changing the concentration of Ca²⁺,Mg²⁺ and CO₃ ²⁻ ions used in the synthesis, CaCO₃ orCa_(1-x)Mg_(x)(CO₃)₂ spheres with different sizes in 1-25 μm can beobtained.

FIG. 3A is an SEM image of calcium carbonate spheres synthesized inaccordance with the microemulsion method described in reference to block202 of FIG. 2A. The spheres of FIG. 3A have a diameter of about 10 μm,which provides pores of about 2.25 μm to about 4.14 μm in size (about22.5% to 41.4% of the size of the spheres) in a random close packingconfiguration. The SEM image of FIG. 3A was taken by scanning electronmicroscopy (SEM, JEOL 7100) at 15 kV, and no additional coating wasapplied onto the sample surface.

In an embodiment of the present disclosure, a microemulsion can be usedas a template medium to synthesize calcium carbonate particles withsizes varying from 20 nm-2000 nm with a narrow size distribution.

A microemulsion medium can be comprised of:

Igepal CO-720 (surfactant), 27.5 g

Hexanol (cosurfactant), 22 mL

H₂O, 13.75 mL

Cyclohexane, 170 mL

In a synthesis process in accordance with this embodiment, calciumchloride (CaCl₂) was dissolved in one part of the microemulsion andsodium carbonate (Na₂CO₃) was dissolved in another part of themicroemulsion. The two parts of the microemulsions were rapidly mixedwith vigorous stirring with a 1:1 Ca²⁺ to CO₃ ²⁻ molar ratio. Then, withmild stirring, the reaction completes in approximately two (2) hours andthe formed particles can be separated from the microemulsion and washedby water and ethanol in turn to remove adsorbed surfactant andimpurities. Finally, the purified calcium carbonate particles areredispersed in absolute ethanol. The shape of resulting calciumcarbonate particles is spherical with a narrow size distribution.

FIG. 3B is another SEM image of calcium carbonate spheres synthesized inaccordance with the microemulsion method described in reference to block202 of FIG. 2A. The spheres of FIG. 3B have a diameter of about 1000 nm,which provides pores of about 225 nm to about 414 nm in size in a randomclose packing configuration. The SEM image of FIG. 3B was taken byscanning electron microscopy (SEM, JEOL) at 15 kV, and no additionalcoating was applied onto the sample surface.

Calcium carbonate spheres with sizes from about 50 nm to 10 μm can alsobe synthesized by reaction of Ca′ ions in mixed solvent (such as DMF-H₂Oor alcohol-H₂O) with or without polymer as stabilizer, and vapor ofsolid (NH₄)₂CO₃.

In an embodiment of the present disclosure, 10 mg polymer polyacrylicacid (PAA, MW=2000) and 1 mL of 0.1M CaCl₂ aqueous solution were mixedin 10 mL isopropanol in a reaction vessel (50 mL beaker).

The reaction vessel was covered with parafilm, which was punctured with6 needle holes, and placed into a desiccator at room temperature. APetri dish (OD: 10 cm) filled with 5 g crushed ammonium carbonate andcovered with parafilm punctured with 10 needle holes were also placedaround the beaker in the closed desiccator. The parafilm is then removedand the precipitates in beaker were collected by centrifuge and rinsedwith DI water and ethanol, and then allowed to dry at room temperature.

FIG. 3C is an SEM image of calcium carbonate spheres synthesized inaccordance with the PAA crystal modifier method described in referenceto block 202 of FIG. 2A, with a solvent mixture of DMF:H2O=1:1. Thespheres of FIG. 3C have a diameter of about 200 nm, which provides poresof about 45 nm to about 82.5 nm in size in a random close packingconfiguration. The SEM image of FIG. 3C was taken by scanning electronmicroscopy (SEM, JEOL) at 15 kV, and no additional coating was appliedonto the sample surface.

FIG. 3D is an exemplary Raman spectrum (Horiba, LabRAM HR Evolution) ofthe calcium carbonate spheres synthesized in accordance with thesolution precipitation method. The CaCO₃ spheres prepared by thesolution-vapor reaction method and the microemulsion template methodalso exhibit the same vibrational bands in their Raman spectra.

Synthesis of Sodium Chloride Crystals

Laboratory-synthesized sodium chloride crystals are typically cubic inshape. By using a supersaturated solution and alcohol to crystalizesodium chloride, the size of the crystals can be controlled. Sodiumchloride can be first dissolved in hot water until saturateconcentration is reached and then cooled down to room temperature toform a supersaturated solution. Upon introducing alcohol to thesolution, crystals of sodium chloride precipitate gradually. The sizesof the sodium chloride crystals depend on the amount and type ofalcohols added to the solution. Available alcohols include, for example,ethanol, n-propanol, isopropanol, n-butanol, 2-butanol or pentanol, andresulting sizes of NaCl cubes can be controlled in the range of 500nm-500 μm. Sodium chloride crystals of the desired size can be separatedand collected by centrifuge.

FIGS. 4A-4C are SEM images of sodium chloride (NaCl) crystalssynthesized in accordance with the method of controlled crystallizationfrom alcoholic solvent. The crystals of FIGS. 4A-4C are substantiallycubic in shape and have a controlled width of about 1-300 The SEM imagesof FIG. 4A-4C were taken by scanning electron microscopy (SEM, JEOL7100) at 15 kV, and no additional coating was applied onto the samplesurface.

In this disclosure, the terms “a,” “an,” or “the” are used to includeone or more than one unless the context clearly dictates otherwise. Theterm “or” is used to refer to a nonexclusive “or” unless otherwiseindicated. The statement “at least one of A and B” has the same meaningas “A, B, or A and B.” In addition, it is to be understood that thephraseology or terminology employed in this disclosure, and nototherwise defined, is for the purpose of description only and not oflimitation. Any use of section headings is intended to aid reading ofthe document and is not to be interpreted as limiting; information thatis relevant to a section heading may occur within or outside of thatparticular section.

In this disclosure, “approximately” or “substantially” means a deviationor allowance of up to 10 percent (%) and any variation from a mentionedvalue is within the tolerance limits of any machinery used tomanufacture the part. Likewise, “about” can also allow for a degree ofvariability in a value or range, for example, within 10%, within 5%, orwithin 1% of a stated value or of a stated limit of a range.

Values expressed in a range format should be interpreted in a flexiblemanner to include not only the numerical values explicitly recited asthe limits of the range, but also to include all the individualnumerical values or sub-ranges encompassed within that range as if eachnumerical value and sub-range is explicitly recited. For example, arange of “0.1% to about 5%” or “0.1% to 5%” should be interpreted toinclude about 0.1% to about 5%, as well as the individual values (forexample, 1%, 2%, 3%, and 4%) and the sub-ranges (for example, 0.1% to0.5%, 1.1% to 2.2%, 3.3% to 4.4%) within the indicated range. Thestatement “X to Y” has the same meaning as “about X to about Y,” unlessindicated otherwise. Likewise, the statement “X, Y, or Z” has the samemeaning as “about X, about Y, or about Z,” unless indicated otherwise.

While this disclosure contains many specific implementation details,these should not be construed as limitations on the subject matter or onwhat may be claimed, but rather as descriptions of features that may bespecific to particular implementations. Certain features that aredescribed in this disclosure in the context of separate implementationscan also be implemented, in combination, or in a single implementation.Conversely, various features that are described in the context of asingle implementation can also be implemented in multipleimplementations, separately, or in any suitable sub-combination.Moreover, although previously described features may be described asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can, in some cases, beexcised from the combination, and the claimed combination may bedirected to a sub-combination or variation of a sub-combination.

Particular implementations of the subject matter have been described.Nevertheless, it will be understood that various modifications,substitutions, and alterations may be made. While operations aredepicted in the drawings or claims in a particular order, this shouldnot be understood as requiring that such operations be performed in theparticular order shown or in sequential order, or that all illustratedoperations be performed (some operations may be considered optional), toachieve desirable results. Accordingly, the previously described exampleimplementations do not define or constrain this disclosure.

What is claimed is:
 1. A mixed-porosity microfluidic chip, comprising: amicrofluidic chip comprising a microchannel etched in a substrate;microporous structures comprising calcium carbonate spheres sintered inthe microchannel; macropores between at least some of the microporousstructures, wherein the average width of the macropores is at leastabout ten times larger than the average diameter of the calciumcarbonate spheres.
 2. The mixed-porosity microfluidic chip of claim 1,wherein the microporous structures comprise micropores between thecalcium carbonate spheres, and wherein the average width of themacropores is at least about ten times larger than the average width ofthe micropores.
 3. The mixed-porosity microfluidic chip of claim 1,wherein the calcium carbonate spheres are from about 25 nanometers toabout 25 microns in diameter.
 4. The method of claim 1, wherein themacropores have a width of about 10 microns to about 250 microns.
 5. Themixed-porosity microfluidic chip of claim 1, wherein the mixed-porositymicrofluidic chip is optically transparent or translucent.
 6. Themixed-porosity microfluidic chip of claim 1, wherein the microchannelhas a width of from about 500 microns to about 1500 microns and a heightof from about 50 microns to about 500 microns.
 7. The mixed-porositymicrofluidic chip of claim 1, wherein the macropores are substantiallycubic in shape.